This invention relates to the fabrication of three-dimensional objects using extrusion-based layered manufacturing techniques. More particularly, the invention relates to a deposition modeling system utilizing a feed drive mechanism that accommodates filament having various diameters, more effectively controls movement of the filament and is easier to service and repair.
Three-dimensional models are used for functions including aesthetic judgments, proofing a mathematical computer aided design (CAD) model, forming hard tooling, studying interference and space allocation, and testing functionality. Extrusion-based layered manufacturing machines build up three-dimensional models by extruding solidifiable modeling material from an extrusion head in a predetermined pattern, based upon design data provided from a CAD system. A feedstock of either a liquid or solid modeling material is supplied to the extrusion head. One technique is to supply modeling material in the form of a filament strand. Where the feedstock of modeling material is in solid form, a liquifier brings the feedstock to a flowable temperature for deposition.
Examples of extrusion-based apparatus and methods for making three-dimensional objects are described in Valavaara U.S. Pat. No. 4,749,347, Crump U.S. Pat. No. 5,121,329, Crump U.S. Pat. No. 5,340,433, Crump et al. U.S. Pat. No. 5,503,785, Danforth, et al. U.S. Pat. No. 5,900,207, Batchelder, et al. U.S. Pat. No. 5,764,521, Batchelder, et al. U.S. Pat. No. 5,968,561, Dahlin, et al. U.S. Pat. No. 6,022,207, Stuffle et al. U.S. Pat. No. 6,067,480 and Batchelder, et al. U.S. Pat. No. 6,085,957, all of which are assigned to Stratasys, Inc., the assignee of the present invention.
In the modeling machines employing a filament feed, modeling material is loaded into the machine as a flexible filament wound on a supply reel, such as disclosed in U.S. Pat. No. 5,121,329. A solidifiable material which adheres to the previous layer with an adequate bond upon solidification and which can be supplied as a flexible filament is used as the modeling material. The extrusion head, which includes a liquifier and a dispensing nozzle, receives the filament, melts the filament in the liquifier, and extrudes molten modeling material from the nozzle onto a base contained within a build envelope. The modeling material is extruded layer-by-layer in areas defined from the CAD model. The material being extruded fuses to previously deposited material and solidifies to form a three-dimensional object resembling the CAD model. In building a model from a modeling material that thermally solidifies upon a drop in temperature, the build envelope is preferably a chamber which is heated to a temperature just below the solidification temperature of the modeling material during deposition, and then gradually cooled to relieve stresses from the material. As disclosed in U.S. Pat. No. 5,866,058, this approach anneals stresses out of the model while it is being built so that the finished model is stress free and has very little distortion.
In creating three-dimensional objects by depositing layers of solidifiable material, supporting layers or structures are built underneath overhanging portions or in cavities of objects that are under construction and are not supported by the modeling material itself. For example, if the object is a model of the interior of a subterranean cave and the cave prototype is constructed from the floor towards the ceiling, then a stalactite will require a temporary support until the ceiling is completed. A support structure may be built utilizing the same deposition techniques and apparatus by which the modeling material is deposited. The apparatus, under appropriate software control, produces additional geometry acting as a support structure for the overhanging or free-space segments of the object being formed. Support material is deposited either from a separate dispensing head within the modeling apparatus, or by the same dispensing head that deposits modeling material. A support material is chosen that will adhere to the modeling material during construction, and that is removable from a completed object. Various combinations of modeling and support materials are known, such as are disclosed in U.S. Pat. No. 5,503,785.
In Stratasys FDM® three-dimensional modeling machines of the current art which embody a filament feed as disclosed in the above-referenced patents, a coil of modeling filament wrapped on a spool is loaded into the machine by mounting the spool onto a spindle. The filament is made of a thermoplastic or wax material. The user may manually feed a strand of the filament through a guide tube made of low friction material, unwinding filament from the spool until the filament strand reaches a pair of motor-driven feed rollers at the extrusion head. Conversely, a series of feed rollers may be utilized and positioned beginning at the spool or source of filament, along a feed path and ultimately at the extrusion head to advance the strand of filament within the modeling machine. The force required to advance, drive, or feed the strand of filament can be substantial at times and require as much as approximately twenty pounds of push force.
The filament strand is ultimately advanced by the feed rollers into a liquifier carried by the extrusion head. Inside the liquifier, the filament is heated to a flowable temperature. As the feed rollers continue to advance filament into the extrusion head, the force of the incoming filament strand extrudes the flowable material out from the dispensing nozzle where it is deposited onto a substrate that is removably mounted to a build platform. The flow rate of the material extruded from the nozzle is a function of the rate at which the filament is advanced to the head and the size of the dispensing nozzle orifice. Pieces of contaminants may also exist in the liquifier and are extruded out of the nozzle along with the filament. Depending upon the size of the piece of contaminant, it may partially or completely plug the nozzle creating significant pressure peaks on the order of four to five times greater than the pressure level normally contained in the liquifier. The feed rollers have to be able to continue to drive the filament during these pressure peaks.
A controller controls movement of the extrusion head in a horizontal x, y plane, controls movement of the build platform in a vertical z-direction, and controls the rate at which the feed rollers advance filament into the head. By controlling these processing variables in synchrony, the modeling material is deposited at a desired flow rate in “beads” or “roads” layer-by-layer in areas defined from the CAD model. The dispensed modeling material solidifies upon cooling, to create a three-dimensional solid object.
In order for the controller to properly control the movement of the extrusion head to create the desired defined three-dimensional solid object, the feed rollers must consistently feed or drive the filament at a predetermined rate without filament slippage. Slippage of the filament in the feed rollers occurs when the filament fails to advance at the desired rate despite rotation of the feed rollers. Filament slippage creates a shortage of modeling material that is supplied to the extrusion head that in turn creates a shortage of modeling material to build the desired three-dimensional model at the defined time and location. This can result in an unsuccessful build of the three-dimensional model that may not be as structurally sound as the designed part or may be deformed due to the shortage of modeling material during the building process.
In an effort to prevent filament slippage, feed rollers have been manufactured out of urethane and mounted on fixed axles to compress against and advance or drive the strand of filament. However, time and varying diameters of strands of filament driven by the feed rollers cause wear on the urethane feed rollers which ultimately lead to an inability of the feed rollers to apply sufficient compression on the strand of filament for consistent advancement under varying conditions. In particular, as the required push force to advance the filament approaches twenty pounds of pressure, it often exceeds the amount of force that urethane feed rollers are able to apply to the filament resulting in the filament slipping and not advancing at the desired rate if even at all.
Thus, there exists a need to provide a drive mechanism to accommodate filaments with varying diameters while more effectively driving the feed rollers to advance the filament.